As information output apparatuses in wordprocessors, personal computers, facsimiles, and the like, printing apparatuses for printing information such as desired characters and images on sheet-like printing media such as paper sheets and films are widely used.
These printing apparatuses are currently used as printers in business offices, clerical work departments, and personal use. Along with strong demands for higher density and higher printing speed, developments and improvements of printing apparatuses have been made to achieve further cost down, higher resolution, and the like.
Of printing apparatuses, an ink-jet printing apparatus designed to perform low-noise, non-impact printing to print images by discharging ink from orifices arranged in printing elements is capable of performing high-density, high-speed printing and widely used as a low-cost color printer owing to its structural feature.
A typical ink-jet printing apparatus is adapted to print an image by discharging ink in accordance with desired printing information using a printhead including printing elements (nozzles) each having an orifice and an electrothermal transducer for generating discharging energy for discharging ink from the orifice.
Conventionally, various types of printheads having a plurality of printing elements arrayed in a line are known. A printhead of this type can arbitrarily print images on a printing material (printing medium) such as paper by having several or several ten driving integrated circuits, each capable of simultaneously driving N printing elements as one block, mounted on a single substrate and arraying image data in correspondence with the respective printing elements.
In such a printhead, as the number of printing elements to be simultaneously driven increases, the power required for driving increases, posing problems in terms of the capacity of a power supply circuit and cost. In addition, when printing elements designed to perform printing by using heat are used, if one printing element is continuously driven, heat is accumulated. As a consequence, the printing density may change or the printing element itself may be destroyed.
Each printing element is also affected by heat from adjacent printing elements. For example, in an ink-jet printing apparatus, when adjacent printing elements are simultaneously driven, the respective nozzles are interfered with each other by mutual pressures produced in discharging ink. In some case, the printing density changes due to this pressure interference (crosstalk). For this reason, a halt period is preferably set after driving of printing elements so as to dissipate heat to a certain degree or avoid crosstalk.
As a driving method that can cope with the above problems and requirements, a method of grouping the printing elements into a plurality of blocks and time-divisionally driving the printing elements in units of blocks is known. In addition, a distributed driving method is known, in which printing elements to be simultaneously driven are distributed in the array direction such that adjacent printing elements belong to different blocks.
According to this driving method, since adjacent printing elements are not simultaneously driven, the influences of adjacent printing elements can be eliminated by setting a halt period.
FIG. 14 is a circuit diagram showing a specific example of a circuit arrangement for time-divisionally driving printing elements, in units of blocks, which perform printing by using heat. FIG. 15 is a timing chart of signals input to the circuit in FIG. 14.
Referring to FIG. 1, reference numeral 1 denotes an electrothermal transducer such as a heater provided for each printing element; 2, a functional element such as a transistor or FET for controlling the energization state of each electrothermal transducer; 3, an AND circuit for outputting a control signal for each functional element; 5, a decoder; 9, a power supply line; 10, a ground line; 13, a shift register, and 14, a latch. Reference symbol CLK denotes a clock signal; DATA, an image data signal; LAT, a latch pulse; BENB, a block selection signal; and ENB, a driving pulse signal.
When the image data signal DATA is input, image data are sequentially transferred to the shift register 13 by the image data transfer clock CLK and arrayed in the latch 14 in correspondence with the respective printing elements. Time-divisional driving can be performed by sequentially activating the block selection signals BENB within a period of the latch pulse signal LAT. In this case, if the block selection signals BENB are distributedly connected to printing elements, distributed driving is performed.
In addition, a printing apparatus having various printing modes uses a method of changing the pattern of the block selection signals BENB to be input to the decoder 5 within a period of the latch pulse signal LAT in accordance with a printing mode. In this case, printing elements can be driven in various patterns by combining other control signals.
In the printing apparatus using the above printhead, however, in order to increase the printing speed or printing density, the number of printing elements arranged in the printhead increases, and the density of printing elements also, increases. For this reason, the number of blocks in the above time-divisional driving method increases, and the number of control signal lines increases even with the use of decoder circuits and the like. This tendency is typical when the driving pattern is changed in accordance with a printing mode.
To increase the printing speed, the transfer clocks for image data are also speeded up. This is because image data corresponding to all the printing elements on the printhead must be transferred within a period of a latch pulse.
A printhead circuit like the one described above is often manufactured as one chip-like heater board (H. B.) by using a semiconductor manufacturing process. If, however, the transfer clock for image data is set to 10 MHz or more, the buffer size in the integrated circuit increases owing to high-speed transfer, although it depends on the semiconductor design rule.
For example, in the circuit arrangement shown in FIG. 14, the number of shift registers and latch circuits corresponding to the respective printing elements becomes enormous. As a consequence, the cost of the printhead incorporating this circuit increases. A method of simply setting a plurality of input lines for image data is known. In this case, however, since data are serially transferred at a high frequency, problems associated with the connection between the printhead and the printing apparatus and an increase in driving current and secondary problems associated with radiation noise and the like arise.
In serial scan type printing apparatuses for general personal use as well, printheads having a print width of 1 inch become increasingly popular than those having a print with of 0.5 inches. In, for example, a printhead with 300 nozzles/inch, if it takes 0.1 μs (10 MHz) per pixel, a time t required to transfer 1-line image data for the printhead is given byt=0.1×300=30 [μsec]
As is obvious from the timing chart of FIG. 15, this time greatly influences the driving period of each printing element. To drive each printing element at a high speed, the period until the start of image data transfer of the next line must be shorter than the above time. For this purpose, a technique of shifting data at both the leading edge and trailing edge of a clock is implemented. In this case as well, image data must be transferred to the printhead at a high speed.
For a printhead with 600 nozzles/inch, for example, to cope with this problem, the driving speed of each printing element is decreased in inverse proportion to an increase in the number of printing elements.
As another method of increasing the driving speed, a method of dividing a printing element array into a plurality of portions, and using a plurality of input lines for image data is available. This method is implemented for an elongated printhead of a full-line type and the like. However, several to ten-odd image data input lines must be prepared in accordance with the number of nozzles of the printhead. With this arrangement, driving circuits with different specifications must be designed again for the respective printheads with different specifications.
In performing multi-grayscale printing, feedback control for a temperature sensor or the like is performed, and complicated driving control is performed to, for example, increase the pulse width for energization of each printing element or continuously applying double pulses or short pulses. In this case as well, the number of signal lines for driving printing elements increases.
If the number of signal lines increases in this manner, the number of connection terminals between the printhead and the printing apparatus body increases, causing various problems. For example, the cost of the connector portions of the printhead and apparatus body increases, and contact failures occur at connection portions. In an ink-jet printing apparatus, an operation error may be caused when ink adheres to the connector portions.
An arrangement in which a plurality of printing elements are assigned to each pixel to achieve high resolution is also known. In this case as well, it is inevitable that the number of control signals for driving the respective printing elements and the circuit size will increase.